Ink-jet technology using aqueous inks is very well understood when jetting fluids of that exhibit Newtonian fluid dynamic behavior. However, high solids containing fluid formulations are particularly troublesome during an initial fluid jetting startup after a period of non-ejection of fluid. For example, ink-jet inks may contain high density particles such as colorant and/or pigments which may settle from the bulk fluid quickly in the fluid ejection heads. Settling of such high density particles causes a concentration gradient which significantly increases the fluid density and viscosity in the microscopic flow features of a fluid ejection head.
Accordingly, such fluids will often fail to eject from fluid jet heads when the jet head ejection nozzles or jet heads are idle for a period of time, thus resulting in poor startup of fluid ejection, poor fluid jetting, and in the case of colorants and/or pigments, poor color consistency. While the bulk fluid may be stirred or agitated to increase the dispersion of particles in the fluid, such stirring or agitation is not effective for the flow feature areas and nozzles in the ejection head. Accordingly, the initial ejection of high solids containing fluids from an ejection head is a challenge.
By “high solids” is meant fluid formulations that contain from about 8 wt. % to about 25 wt. % solid particles. Such fluids may be used in ink formulations for cosmetic ink applications, for example, and thus may have a relatively high non-volatile particle settling rate over time and a viscosity of above about 1 Pa-sec at 25° C.
Embodiments of the disclosure provide an ink composition, a printer and a method for ejecting and aqueous fluid having a viscosity above about 1 Pa-sec at 25° C. from a fluid ejection head. The ink composition, printer and method include an aqueous fluid that contains an aqueous carrier component, solid particles ranging from about 8 wt. % to about 25 wt. % based on a total weight of the aqueous fluid, and a rheology modifier. A heater pulse signal applied to ejection heaters on a fluid ejection head for a period of time is sufficient to sputter fluid from fluid ejection nozzles associated with the ejection heaters and to shear the fluid thereby reducing the viscosity of the fluid from above about 1 Pa-sec to less than about 0.1 Pa-sec at 25° C. A firing signal subsequently applied to the ejection heaters on the ejection head provides steady state fluid ejection from the ejection head.
In one embodiment, there is provided an ink composition that includes water, pigment particles, and a rheology modifier. The ink composition has a pigment content ranging from about 8 wt. % to about 25 wt. % based on a total weight of the ink composition. The rheology modifier is an alkali swellable acrylic polymer emulsion having a solids content of about 30 wt. % and a pH below about 4 and the rheology modifier is effective to provide a pseudo-plastic aqueous ink formulation.
A further embodiment provides a printer that contains an aqueous ink composition having a viscosity of greater than about 1 Pa-sec at 25° C. The aqueous ink composition includes, an aqueous carrier component, solid color particles ranging from about 8 wt. % to about 25 wt. % based on a total weight of the aqueous ink composition, and a rheology modifier. A heater pulse signal applied to ejection heaters on an ejection head of the printer for a period of time is sufficient to sputter ink from ejection nozzles associated with the ejection heaters and to shear the aqueous ink composition thereby reducing the viscosity of the ink composition from above about 1 Pa-sec to less than about 0.1 Pa-sec at 25° C. A firing signal applied to the ejection heaters on the ejection head provides steady state ejection of the aqueous ink composition from the ejection head.
In some embodiments, the rheology modifier is an alkali swellable acrylic polymer emulsion having a solids content of about 30 wt. % and a pH below about 4. In other embodiments, the aqueous fluid or ink contains from about 0.2 to about 1.0 percent by weight of the rheology modifier based on a total weight of the aqueous fluid or ink. In some embodiments, the rheology modifier provides a pseudo-plastic ink-jet ink formulation that, upon shearing, has a reduction in viscosity from about 1 Pa-sec to less than about 0.1 Pa-sec at 25° C. Such fluids may be hereinafter referred to as “thixotropic” fluids. Such fluids may be characterized by a thixotropic index. The thixotropic index represents the degree of thixotropy and is determined by taking a ratio of static viscosity of the fluid to dynamic viscosity of the fluid.
In some embodiments, the heater pulse signal is at a frequency ranging from about 2 to about 5 KHz for a period of three to five seconds. In other embodiments, the heater pulse signal has a pre-heat pulse of about 200 to about 400 nanoseconds (nsec), a dead time of about 1200 nsec and a firing pulse ranging from about 900 to about 1000 nsec.
In some embodiments, the heater pulse signal is effective to provide a shear rate of the aqueous fluid or ink within flow features of the ejection head ranging from about 2 to about 3×106 per second.
In other embodiments, fluid or ink ejection is initiated in the absence of a pre-heat pulse used to lower the viscosity of the fluid or ink.
The foregoing methods are particularly suitable for the initial ejection of fluids having a high solids content when the ejection head has been idle or unused for a period of time. Such fluids typically contain thickeners or emulsifiers that tend to keep the solids in suspension. However, such thickeners or emulsifiers tend to greatly increase the viscosity of the fluids while keeping the particles in suspension in the fluids. Accordingly, it is difficult to refill the ejection heads after long idle times due to the size of flow features in the ejection head and the high viscosity of such fluids. The foregoing embodiments enable start up and continued operation of ejection heads ejecting such fluids without the need for a preheat pulse or step that may be harmful to the fluids and may accelerate drying of the fluid in or on the nozzles causing plugged nozzles.